One of the many uses for hydrogen peroxide solutions, and especially aqueous acidic hydrogen peroxide solutions, comprises the treatment of metal surfaces so as to alter their appearance and to impart chemically to the surface a desired sheen or polish. This is often referred to simply as pickling or polishing. Conventionally, solutions for that use contain one or more strong acids, which, is normally a mineral acid, as well as the hydrogen peroxide. In the course of the metal surface treatment, there is a tendency for the solution to dissolve metal or impurities from the metal surface and to strip away particulate particles that had adhered to the metal surface before the treatment commenced. The metals that are pickled or polished usually comprise or contain at least a proportion of transition metals, such as iron or copper, which catalyse the wasteful decomposition of hydrogen peroxide in aqueous solution into oxygen and water.
In view of its decomposition in situ, hydrogen peroxide often represents the major consumable cost in a pickling or polishing process. In consequence, the industry continues to seek ever more effective ways of reducing the rate and/or extent of the decomposition. In many instances, it has been sought by introducing into solution one or more substances that are often called stabilisers, which interact with the metal ions and/or metal surface and/or the hydrogen peroxide itself in such a way as to reduce the rate or extent or modify the manner of the interactions between the metal ions and hydrogen peroxide causing decomposition.
There have been many different chemical types of stabilisers proposed or employed. The literature directed to peroxide stabilisation during metal surface treatment processes includes many organic compounds as stabilisers such as a range of organic acids or unsaturated aliphatic acids in U.S. Pat. No. 3,537,895 by L. E. Lancy, aromatic alcohols or unsaturated aliphatic alcohols in U.S. Pat. No. 3,869,401 by R. E. Ernst, saturated alcohols in U.S. Pat. No. 3,556,883 by A. Naito et al, amines, amides and imines in U.S. Pat. No. 3,756,957 by S. Shiga, aryl sulphonic or sulphamic acids or related compounds in U.S. Pat. No. 3,801,512 by J. C. Solenberger et al and solid poorly soluble stabilisers like hydroxybenzoic acid in U.S. Pat. No. 4,770,808 by C. F. McDonogh et al. Many other stabilisers have been suggested for peroxide solutions including substances that chelate the metal ions or precipitate them out of solution, for example in U.S. Pat. No. 4,059,678 to D. C. Winkley. The literature also includes references to inorganic substances, such as phosphoric acid in U.S. Pat. No. 3,373,113 to Achenback. Accordingly there is a wide pool of stabilisers from which the user can select.
Despite the foregoing, the present inventors found that there remains a significant problem of stabilising hydrogen peroxide during the metal surface treatment of steel with aqueous sulphuric acid solutions of hydrogen peroxide. This is because the greater part of the literature was directed to the treatment of copper surfaces and the authors extrapolated to the treatment of other metals without adequate experimental support. To some extent, this is demonstrated in U.S. Pat. No. 3,407,141 to R. S. Banush et al, which seeks to etch copper with acidic hydrogen peroxide solutions of long storage life that contain certain urea and aromatic acid compounds. The specification suggests that the treatment can be applied to certain other metals but also that the solutions are less effective on certain other metals such as . . . stainless steel . . . . Since the patent disclosed results solely with copper, comments regarding other metals may be regarded simply as speculation.
In the course of the present research to identify, if possible, a suitable stabiliser system for acidic hydrogen peroxide solutions which are severely contaminated with dissolved iron, resulting for example from the surface treatment of steels, a large number of comparative stability trials were conducted. Each trial employed a stock solution containing 180 g/l sulphuric acid and 50 g/l hydrogen peroxide and 1% w/w "stabiliser" which was contaminated with 25 g/l dissolved iron from ferric sulphate, and stored at 30.degree. C. or 50.degree. C. Many of the substances tested fell within the classes of stabilisers identified in the above-mentioned patent specifications.
The trials indicated that many substances which had been described in the past as stabilisers for hydrogen peroxide in solutions containing only small amounts of catalytic ions, were unable to prevent rapid decomposition if substantial iron contamination was present, including chelating stabilisers like ethylenediaminetetraacetic acid, dipicolinic acid, nitrilotriacetic acid and ethylidene-1-hydroxy-1,1-diphosphonic acid. Moreover, it was found that some substances that acted quite well as stabilisers when employed separately, acted no better or even less well when employed in cogitation under the conditions of the trial. Other combinations of substances demonstrated strictly additive stabiliser properties. Accordingly, the trials demonstrated that a disclosure in a published patent specification that a substance had stabiliser properties towards hydrogen peroxide under much less extreme conditions or in the presence of copper as the main catalytic contaminant was no guarantee that it was capable of performing adequately in the presence of a substantial concentration of dissolved iron. The trials also demonstrated that there was no guarantee that substances that had been suggested individually as stabilisers, possibly guarding against other sources of decomposition, would combine together even additively when employed in combination.